I have to admit I like all the electronic navigation instruments you can get for your boat. However, the compass, VHF radio, and depth person are the three instruments which will be used the most and they provide a link of safety. In the middle of the day, on open water and out of view of land where you have no points of interest you can see, a compass can tell you what direction land is. That is a scary situation. Even if you never lose view of land, it still can help you get your location going and back again.
There are two types of compasses, over unity magnetic and electronic. The over unity magnetic type has been online for years and years. They are reliable monetization of leased bank instruments. and accurate and come in different styles and supports that let you pick one that would work for the type of boat you have and where you can mount it. The digital or electronic compasses available nowadays are also reliable and accurate and they can be integrated with other navigation instruments easily. That is their biggest asset.
Buy the best compass you can afford. The compass lasts for years and it is an investment safely and peace of mind. Pick the largest, easiest to see compass you have space for. If it is hard to see, it can be bothersome and dangerous. There are a wide variety of prices and styles on the market to pick from and making the selection can be fun. Pick the best you can afford.
A VHF radio is the next instrument you need. They can be hand-held or mounted, and come in an extensive price range. Hand-held radios have a shorter range than the usual mounted radio with a separate antenna. As a rule of thumb, a hand-held has a brand of view range. A radio lets you talk to other boats, get directions, talk to draw bridge operators and lock attendants, and call for help if needed. A cell phone can do most of these if you are near a cell system, but cell phones are not administered by the Shoreline Guard and a VHF signal is. My travel boat has an antenna on the mast and that can give a fantastic range to the VHF radio. We use a hand-held for most things since it can be in the cockpit at hands reach. The hand-held is more than adequate for most situations, and it was less expensive than the mounted unit. Again, get the best your finances will let you afford.
Depth finders is able to keep you out of trouble. You cannot often see low spots in open water or where a funnel is but a depth person can. How much water your boat and motor draw limits where you can go and if you will get stuck. There are BIG penalties now for damaging some seabed’s if you get caught. A depth person can help prevent you from getting grounded.
Like most navigation instruments, depth finders come in a wide range of prices and are often multifunctional. You can get a one that just reads depth, one that is a depth and fish person combination, or one that is a depth greater, fish person and GPS-chart plotter all in one. Get what you can afford. My boat has just a depth person that has a warning signal that lets me know it is getting low. This is more than adequate, and that is what my 20-year-old boat sported. The fish person that we have provides the same information but graphically and can are more useful seeing the bottom shape. It also sees fish. 8-). If just fitting out a boat I would go with a depth person or fish person unit. Screen size needs to be considered. If it is too small, it is hard to use. Focusing on a small screen takes your focus off from steering.